Recently, as a part of the development of a submicron scale semiconductor device which has appeared as a result of the rapid progress of the semiconductor technology a trend is seen in the art where the supply voltage for a CMOS device is stepped down from 5V to 3.3V to assure the proper operation of such a submicron scale semiconductor device. Accordingly, it is frequently seen that the structure of the semiconductor chip is designed such that it can provide the alternative of 5V or 3V for the power source.
A conventional circuit is illustrated in FIG. 1 wherein two sources of power can be alternatively used by translating the TTL level signal applied to the input terminal of the semiconductor chip to a CMOS logic level.
As will be discussed in detail hereinbelow, the conventional circuit (FIG. 1) includes at least one CMOS inverting means having a p-channel MOS transistor and a n-channel transistor connected in series and receiving an external input signal of the TTL levels. The circuit also includes at least one p-channel MOS transistor for receiving enable signals into the gate thereof so as to allow the CMOS inverting means to have a tri-state condition in response to the enable signals. Furthermore, as specifically shown in the identical drawing in conformity with a customer's requirement an additional p-channel MOS transistor is connected in parallel to the p-channel MOS transistor within the CMOS inverting means so that two different power sources may be alternatively utilized.
The additional p-channel MOS transistor includes a gate for optionally receiving the power voltage or the external input signals of the TTL levels by means of optional linkages, the linkage state of which could be permanently realized only through utilization a special mask work in the semiconductor manufacturing process. Therefore, the above physical linkage must not be changed over and force the overall CMOS integrated circuit containing the said input translating circuit to be capable of operating under a only single selected power voltage.
Furthermore, such a conventional scheme raises the costs of manufacture.